The U.S. Postal Service (USPS) has developed standards for the marking of mailpieces that facilitate the automatic sorting and processing of such items. Such mailpiece features include stamps, metermarks, information based indicia (IBI) barcodes (PDF417 and Data Matrix), facing identification marks (FIM), Postal Numeric Encoding Technique (POSTNET) codes, postal alphanumeric encoding technique (PLANET) codes, 4CB codes, and identification (ID) tags. The purpose and use of such features are well known in the art and thus will not be described in detail.
Line scan cameras have been implemented in numerous industrial and commercial settings, such as on high-speed mail sorting systems. An example of a prior art mail sorting system that employed such cameras, as well as several other components, is illustrated in FIG. 1. As shown, the mail sorting system 2 comprised a singulation stage 4, a first indicia detection stage 6, a facing inversion stage 8, a second indicia detection stage 10, a cancellation stage 12, an inversion stage 14, an ID tag spraying stage 16, an image lifting stage 18, and a stacking stage 20. One or more conveyors (not shown) would move mailpieces 19 from stage to stage in the system 2 (from left to right in FIG. 1) at a rate of approximately 3.6-4.0 meters per second.
The singulation stage 4 included a feeder pickoff 22 and a fine cull 24. The feeder pickoff 22 would generally follow a mail stacker (not shown) and would attempt to feed one mailpiece at a time from the mail stacker to the fine cull 24, with a consistent gap between mailpieces. The fine cull 24 would remove mailpieces that were too tall, too long, or perhaps too stiff. When mailpieces 19 left the fine cull 24, they would ideally be in one of four possible orientations, as illustrated by mailpieces 19a-d. 
Each of the first and second indicia detection stages 6, 10 included a pair of indicia detectors 26a-b, 26c-d positioned to check the lower edges (of approximately one inch) of the opposite faces of a passing mailpiece 19 for reactance to ultraviolet (UV) radiation and for FIM marks, and thereby detect indicia at such locations. As used herein, “indicia” refers to any marking on a mailpiece that represents a postage value. If the first indicia detection stage 6 failed to detect any indicia on either lower edge of a given mailpiece, that mailpiece would be inverted by an inverter 9 at the facing inversion stage 10 so as to allow the second indicia detection stage 10 to check the lower one inch edges of the other side of the mailpiece for indicia. As a result, each mailpiece 19 that had detectable indicia thereon ideally ended up positioned with the edge containing the indicia (the “top edge” of the mailpiece) facing downward after it left the second indicia detection stage 10, with at least one of the indicia detectors 26a-d having identified the face of the mailpiece that contained the indicia.
The cancellation stage 12 included a pair of cancellers 28a-b arranged to spray one side of the top edge of the mailpiece (i.e., the side determined to contain the indicia), and thereby cancel the indicia. Following the cancellation stage 12, each mailpiece would be inverted by an inverter 15 at the inversion stage 14 so that the top edge of the mailpiece was made to face upwards. The ID tag spraying stage 16 included a pair of ID tag sprayers 30a-b arranged to spray an ID tag, as needed, along an appropriate one of the two lower edges of the mailpiece, as determined by the facing decision made by the indicia detection stages 6, 10.
The image lifting stage 18 included a pair of line scanning cameras 32a-b that imaged the mailpiece. Each line scanning camera provided a two hundred and twelve pixel per inch (PPI) image for address recognition. An analysis of the accumulated images facilitated a determination of the one of several output bins 34a-g of the stacking stage 20 into which the mailpieces was to be stacked based on certain criteria.